It is known that titanium dioxide can be prepared by means of pyrogenic processes.
For the purposes of the present invention, pyrogenic processes are flame oxidations or flame hydrolyses. In flame oxidation, a titanium dioxide precursor, for example titanium tetrachloride, is oxidized by means of oxygen according to equation 1a. In flame hydrolysis, titanium dioxide is formed by hydrolysis of the titanium dioxide precursor, with the water necessary for the hydrolysis coming from combustion of a fuel gas, for example hydrogen, with oxygen (equation 1b).TiCl4+O2->TiO2+2 Cl2  (equation 1a)TiCl4+2 H2O->TiO2+4 HCl  (equation 1b)
The titanium dioxide is generally present in the crystalline modifications anatase and rutile. As described below, conventional methods of preparation are focused on maximizing the proportion of anatase.
WO 96/06803 describes a process for preparing titanium dioxide by means of combination of flame oxidation and flame hydrolysis. Here, a gaseous titanium tetrachloride and oxygen are mixed in a reaction zone and the mixture is heated in a flame produced by combustion of a hydrocarbon as fuel gas. Titanium dioxide powders which have a high content, up to 100%, of the anatase modification are obtained.
Powder Technology 86 (1996) 87-93 describes a flame oxidation process which is carried out in a diffusion reactor. The rutile content is a maximum 6% of in a range of BET surface areas of about 40-60 m2/g.
DE-A-102004055165 discloses a titanium dioxide powder which is prepared by flame hydrolysis and has a BET surface area of from 20 to 200 m2/g. Although the anatase/rutile ratio is said to be able to be varied in a range from 2:98 to 98:2 at a given surface area, the examples show that the proportion of anatase clearly dominates. In a range of BET surface areas of about 40-50 m2/g in particular, the anatase content is at least 71%.
JP-A-10251021 describes three titanium dioxide powders which can be obtained by flame oxidation and have BET surface areas of 3.2, 28 and 101 m2/g and a rutile content of 97, 32 and 15%, respectively.
Titanium dioxide powder, having a high sintering activity is desirable for use, for example, in the ceramics industry. However, due to the complexity of pyrogenic processes, control is limited to only a few product physical parameters.